1. Field of the Invention
The present invention generally relates to integrated circuits and, more specifically, to the non-volatile storage in an integrated circuit of a coding set on manufacturing of the circuit.
2. Discussion of the Related Art
Electrically erasable and programmable read-only memories or EEPROMs are frequently used in integrated circuits. Commercially-available EEPROMs have standard storage capacities. A user needing, for a specific application, a given storage capacity will generally use the standard EEPROM having the storage capacity just greater than the desired storage capacity. Thereby, it is possible for part of the EEPROM not to be used in operation.
Further, in many cases, it is needed to definitely code, on manufacturing of the integrated circuit, digital words in a memory or logic states conditioning the circuit operation. For this purpose, a read-only non-volatile memory or ROM-type memory is generally used.
It would be desirable to be able to modify, simply and at a decreased cost, a portion of an EEPROM to use it as a read-only memory. This would enable obtaining an EEPROM having a portion which would conventionally operate as an electrically erasable and programmable non-volatile memory and having another portion which would operate as a read-only memory. The memory load would thus be optimized.
For conventional read-only memories, the coding is performed by interconnecting the drain and source of transistors forming the memory cells so that the state read from the cell depends on the presence of this connection. The first metallization level of the structure is generally used to perform this programming.
A disadvantage of such a programming is that it is visible by analysis of the mask used to form the interconnection level, for example, by reconstruction of this mask from a circuit (reverse engineering).